(1) Field of the Invention
The present invention relates to a subscriber unit and a radio base station recognition method, and more particularly, to a subscriber unit for performing radio communication at a fixed location and a radio base station recognition method which enables a subscriber unit for performing radio communication at a fixed location to recognize a radio base station.
(2) Description of the Related Art
Currently, in the field of radio communications, attempts have been made on a nationwide scale to effectively use radio frequencies, and digitization and personalization of communications are rapidly spreading. Also, there have been developed a diversity of data transmissions and their related services and demands therefor are more and more increasing.
In recent years, WLL (Wireless Local Loop) is attracting attention which permits radio communications to be established between a subscriber-side exchange and a subscriber's home, which are conventionally connected by wire.
FIG. 24 illustrates a WLL system configuration. The WLL system comprises a public network 40 including an exchange 41, a base station control device 50, and a plurality of zones Z1, Z2a and Z2b. 
The zone Z1 includes a radio base station 101 and a subscriber unit SU11. The zone Z2a includes a radio base station 201 and a subscriber unit SU21, and the zone Z2b includes a radio base station 202 and a subscriber unit SU22. A telephone is connected to each of the subscriber units SU11, SU21 and SU22.
The zones are each assigned an area number associated with location registration and accounting, and the zone Z1 belongs to an area 100 and the zones Z2a and Z2b to an area 200.
The base station control device 50 connects the exchange 41 and each of the radio base stations 101, 201 and 202 by a wireline link and controls the radio base stations 101, 201 and 202. Each of the subscriber units SU11, SU21 and SU22 is connected to a corresponding one of the radio base stations 101, 201 and 202 by a wireless link and performs subscriber radio control for establishing wireless connection between its associated telephone and the corresponding radio base station 101, 201, 202.
As such WLL system, PHS (Personal Handyphone System)-WLL system in particular, which uses a data transmission method complying with PHS, has been developed. Conventional subscriber units therefore have a network-initiated type construction like PHS terminals.
For example, a subscriber unit is so constructed as to operate after a permission level for waiting (state capable of transmission/reception) and a waiting selection level are specified from the network side (base station control device), as is the case with PHS terminals.
The waiting permission level is a prescribed level based on which the subscriber unit is set outside or inside area. If the radio wave from a radio base station satisfies the prescribed level, the subscriber unit is judged to be inside area and thus a call can be established (for example, an antenna mark is displayed on the display screen of the PHS terminal). If the radio wave has a level lower than the prescribed level, then the subscriber unit is judged to be outside area and a call cannot be established because the speech quality lowers due to increased error rate.
The waiting selection level is a prescribed level used to determine whether to search for nearby radio base stations or not. When the radio wave from a radio base station becomes out of synchronism to a degree lower than the prescribed level notified from the network side, the subscriber unit searches for nearby base stations.
In the aforementioned PHS-WLL system, however, since the subscriber unit is a fixed station, it is associated with restrictions that pose no problem with the conventional PHS terminal because of the condition that the conventional PHS terminal is mobile.
For example, in the PHS terminal, if the level of the radio wave drops below the waiting selection level as the PHS terminal moves, the operation for searching for nearby stations is initiated. However, the subscriber unit is not moved, and accordingly, the nearby station search function is not performed. Thus, if the radio base station with which synchroneity is established stops transmitting radio wave, the subscriber unit establishes synchroneity with another base station, but when the original radio base station is restored, the subscriber unit is unable to recognize the restored radio base station.
Also, when a radio base station is additionally installed at a location adjacent to the subscriber unit, the subscriber unit may establish synchroneity with the added radio base station, giving rise to a problem that services cannot be provided to a subscriber who should originally be accommodated.
Further, when the area numbers are rearranged (reassigned), location registration cannot be terminated because the subscriber unit does not have an area number recognition function.